Enantioselective total synthesis of (-)-jiadifenolide

Total syntheses of Tetrodotoxin and 9-epiTetrodotoxin

Tetrodotoxin and congeners are specific voltage-gated sodium channel blockers that exhibit remarkable anesthetic and analgesic effects. Here, we present a scalable asymmetric syntheses of Tetrodotoxin and 9-epiTetrodotoxin from the abundant chemical feedstock furfuryl alcohol. The optically pure cyclohexane skeleton is assembled via a stereoselective Diels-Alder reaction. The dense heteroatom substituents are established sequentially by a series of functional group interconversions on highly oxygenated cyclohexane frameworks, including a chemoselective cyclic anhydride opening, and a decarboxylative hydroxylation. An innovative SmI2-mediated concurrent fragmentation, an oxo-bridge ring opening and ester reduction followed by an Upjohn dihydroxylation deliver the highly oxidized skeleton. Ruthenium-catalyzed oxidative alkyne cleavage and formation of the hemiaminal and orthoester under acidic conditions enable the rapid assembly of Tetrodotoxin, anhydro-Tetrodotoxin, 9-epiTetrodotoxin, and 9-epi lactone-Tetrodotoxin.

Neurotrophic Natural Products

Neurotrophins (NGF, BDNF, NT3, NT4) can decrease cell death, induce differentiation, as well as sustain the structure and function of neurons, which make them promising therapeutic agents for the treatment of neurodegenerative disorders. However, neurotrophins have not been very effective in clinical trials mostly because they cannot pass through the blood-brain barrier owing to being high-molecular-weight proteins. Thus, neurotrophin-mimic small molecules, which stimulate the synthesis of endogenous neurotrophins or enhance neurotrophic actions, may serve as promising alternatives to neurotrophins. Small-molecular-weight natural products, which have been used in dietary functional foods or in traditional medicines over the course of human history, have a great potential for the development of new therapeutic agents against neurodegenerative diseases such as Alzheimer's disease. In this contribution, a variety of natural products possessing neurotrophic properties such as neurogenesis, neurite outgrowth promotion (neuritogenesis), and neuroprotection are described, and a focus is made on the chemistry and biology of several neurotrophic natural products.

Organocatalyst Design for the Stereoselective Annulation towards Bicyclic Diketones and Analogues

The Wieland–Miescher ketone, Hajos–Parrish–Eder–Sauer–Wiechert ketone, and their analogues are bicyclic diketones essential as building blocks for the synthesis of several natural and bioactive molecules. For this reason, since 1971, when Hajos and Parrish and Eder, Sauer, and Wiechert reported the stereoselective synthesis of these compounds promoted by L-proline, numerous methodologies and organocatalysts have been studied over the years with the aim of identifying increasingly efficient asymmetrical syntheses of these bicyclic ketones. This review will outline the methodological and stereochemical features of the organocatalytic stereoselective synthesis of these bicyclic scaffolds based on the different organocatalysts employed from 1971 until today. Particular emphasis will be given to the structural features of the catalysts and to the reaction conditions.

Synthesis of Illisimonin a Skeleton by Intramolecular Diels–Alder Reaction of Ortho-Benzoquinones and Biomimetic Skeletal Rearrangement of Allo-Cedranes

Illisimonin A is a new sesquiterpene isolated from Illicium simonsii, and it possesses a novel 5/5/5/5/5 pentacyclic skeleton. The tricyclic skeleton of illisimonin A, tricyclo[5.2.1.01,5]decane, is presumed to be biosynthesized from allo-cedranes via a skeletal rearrangement. Herein, we report the concise synthesis of highly oxidized allo-cedranes by an intramolecular Diels–Alder reaction using ortho-benzoquinones and demonstrate the biomimetic transformation of allo-cedranes by a retro-Claisen/aldol pathway.

Contemporary advancements in the semi-synthesis of bioactive terpenoids and steroids

Many natural products have intriguing biological properties that arise from their fascinating chemical structures. However, the intrinsic complexity of the structural skeleton and the reactive functional groups on natural products pose tremendous challenges to chemical syntheses. Semi-synthesis uses chemical compounds isolated from natural sources as the starting materials to produce other novel compounds with distinct chemical and medicinal properties. In particular, advancements in various types of sp3 C-H bond functionalization reactions and skeletal rearrangement methods have contributed to the re-emergence of semi-synthesis as an efficient approach for the synthesis of structurally complex bioactive natural products. Here, we begin with a brief discussion of several bioactive natural products that were obtained via a semi-synthetic approach between 2008 and 2015 and we then discuss in-depth contemporary advancements in the semi-synthesis of bioactive terpenoids and steroids reported during 2016-2020.

An invocation for computational evaluation of isomerization transforms: cationic skeletal reorganizations as a case study

Covering: 2010 to 2020This review article describes how cationic rearrangement reactions have been used in natural product total synthesis over the last decade as a case study for the many productive ways by which isomerization reactions are enabling for synthesis. This review argues that isomerization reactions in particular are well suited for computational evaluation, as relatively simple calculations can provide significant insight.

Fragment Coupling Reactions in Total Synthesis That Form Carbon-Carbon Bonds via Carbanionic or Free Radical Intermediates

Fragment coupling reactions that form carbon-carbon bonds are valuable transformations in synthetic design. Advances in metal-catalyzed cross-coupling reactions in the early 2000s brought a high level of predictability and reliability to carbon-carbon bond constructions involving the union of unsaturated fragments. By comparison, recent years have witnessed an increase in fragment couplings proceeding via carbanionic and open-shell (free radical) intermediates. The latter has been driven by advances in methods to generate and utilize carbon-centered radicals under mild conditions. In this Review, we survey a selection of recent syntheses that have implemented carbanion- or radical-based fragment couplings to form carbon-carbon bonds. We aim to highlight the strategic value of these disconnections in their respective settings and to identify extensible lessons from each example that might be instructive to students.

Making natural products from renewable feedstocks: back to the roots?

This review highlights the utilization of biomass-derived building blocks in the total synthesis of natural products.

Diastereoselective Methylation at the Congested β-Position of a Butenolide Ring: Studies toward the Synthesis of seco-Prezizaane-Type Sesquiterpenes

We established a method for installing a methyl group at the β-position of a butenolide ring. The methylated position is located at the congested ring juncture of a 5,6,5-tricyclic lactone, which is common to neurotrophic seco-prezizaane-type sesquiterpenes. The samarium(II)-mediated conjugate addition of the halomethylsilyl ethers tethered to the proximal hydroxy groups efficiently formed the desired C-C bond. Subsequent fluoride-free Tamao oxidation and Barton-McCombie deoxygenation converted the resultant cyclic silyl ether into the corresponding methyl group.

Development of a Terpene Feedstock-Based Oxidative Synthetic Approach to the Illicium Sesquiterpenes

The Illicium sesquiterpenes are a family of natural products containing over 100 highly oxidized and structurally complex members, many of which display interesting biological activities. This comprehensive account chronicles the evolution of a semisynthetic strategy toward these molecules from (+)-cedrol, seeking to emulate key aspects of their presumed biosynthesis. An initial route generated lower oxidation state analogs but failed in delivering a crucial hydroxy group in the final step. Insight gathered during these studies, however, ultimately led to a synthesis of the pseudoanisatinoids along with the allo-cedrane natural product 11- O-debenzoyltashironin. A second-generation strategy was then developed to access the more highly oxidized majucinoid compounds including jiadifenolide and majucin itself. Overall, one dozen natural products can be accessed from an abundant and inexpensive terpene feedstock. A multitude of general observations regarding site-selective C(sp³)-H bond functionalization reactions in complex polycyclic architectures are reported.

Contemporary Synthetic Strategies toward seco-Prezizaane Sesquiterpenes from Illicium Species

Since the elucidation of the structure of anisatin in the late 1960s, sesquiterpene lactones from various Illicium species of plants have captivated synthetic chemists worldwide, resulting in a large body of synthetic work. In particular, Illicium sesquiterpenes containing the seco-prezizaane carbon framework have seen immense interest in recent years owing to desirable structural and medicinal attributes. This synopsis will focus on recently developed synthetic strategies to access these compact, highly oxidized terpenoids.

Enabling strategies for step efficient syntheses

The field of natural product total synthesis has reached the point where synthetic efficiency has become more important than merely defining a viable (yet less ideal) route to the target molecule. Synthetic efficiency is best represented by the number of steps it takes to finish the target molecule from readily available starting materials, as by reducing the number of steps, all other factors of synthetic efficiency are influenced positively. By comparing several total syntheses from the recent years, the most successful strategies for step efficient syntheses will be highlighted. Each synthesis will be presented using a color-coded synthetic flowchart, in which each step is categorized by a colored box. Five categories of transformations are defined and rated according to their synthetic value. Each class will be signified by different colors so that the reader can quickly see which parts of the synthesis are productive and those that are not.

Total Syntheses of (-)-Majucin and (-)-Jiadifenoxolane A, Complex Majucin-Type Illicium Sesquiterpenes

We report the first chemical syntheses of both (-)-majucin and (-)-jiadifenoxolane A via 10 net oxidations from the ubiquitous terpene (+)-cedrol. Additionally, this approach allows for access to other majucin-type sesquiterpenes, like (-)-jiadifenolide, (-)-jiadifenin, and (-)-(1R,10S)-2-oxo-3,4-dehydroxyneomajucin (ODNM) along the synthetic pathway. Site-selective aliphatic C(sp3)-H bond oxidation reactions serve as the cornerstone of this work which offers access to highly oxidized natural products from an abundant and renewable terpene feedstock.

Navigating the Chiral Pool in the Total Synthesis of Complex Terpene Natural Products

The pool of abundant chiral terpene building blocks (i.e., "chiral pool terpenes") has long served as a starting point for the chemical synthesis of complex natural products, including many terpenes themselves. As inexpensive and versatile starting materials, such compounds continue to influence modern synthetic chemistry. This review highlights 21st century terpene total syntheses which themselves use small, terpene-derived materials as building blocks. An outlook to the future of research in this area is highlighted as well.

Synthesis of (-)-11-O-Debenzoyltashironin: Neurotrophic Sesquiterpenes Cause Hyperexcitation

11-O-Debenzoyltashironin (1) is a member of the neurotrophic sesquiterpenes, trace plant metabolites that enhance neurite outgrowth in cultured neurons. We report its synthesis in six steps from a butenolide heterodimer via its likely biosynthetic precursor, 3,6-dideoxy-10-hydroxypseudoanisatin, here identified as the chain tautomer of 1. Access to the tashironin chemotype fills a gap in a comparison set of convulsive and neurotrophic sesquiterpenes, which we hypothesized to share a common target. Here we show that both classes mutually hyperexcite rat cortical neurons, consistent with antagonism of inhibitory channels and a mechanism of depolarization-induced neurite outgrowth.

Synthetic studies en route to the first total synthesis of a naturally occurring quinone from Acorus gramineus, iso-merrilliaquinone, iso-magnoshinin and 2-epi-3,4-dihydro magnoshinin

An elegant Diels–Alder based approach has been demonstrated for the first racemic total synthesis of gramineusquinone B, iso-merrilliaquinone, iso-magnoshinin and 2-epi-3,4-dihydro magnoshinin.

Oxidative Entry into the Illicium Sesquiterpenes: Enantiospecific Synthesis of (+)-Pseudoanisatin

Illicium sesquiterpenes have been the subject of numerous synthetic efforts due to their ornate and highly oxidized structures as well as significant biological activities. Herein we report the first chemical synthesis of (+)-pseudoanisatin from the abundant feedstock chemical cedrol (∼$50 USD/kg) in 12 steps using extensive site-selective C(sp³)-H bond functionalization. Significantly, this work represents a novel oxidative strategic template for future approaches to these natural products and their analogs.

Formal Total Synthesis of (-)-Jiadifenolide and Synthetic Studies toward seco-Prezizaane-Type Sesquiterpenes

Synthetic studies toward highly oxygenated seco-prezizaane sesquiterpenes are reported, which culminated in a formal total synthesis of the neurotrophic agent (-)-jiadifenolide. For the construction of the tricyclic core structure, an unusual intramolecular and diastereoselective Nozaki-Hiyama-Kishi reaction involving a ketone as electrophilic coupling partner was developed. In addition, synthetic approaches toward the related natural product (2R)-hydroxy-norneomajucin, featuring a Mn-mediated radical cyclization for the tricycle assembly and a regioselective OH-directed C-H activation are presented.

Neurite outgrowth enhancement by jiadifenolide: possible targets

Covering: 1860-2016A mechanistic link may exist between convulsant plant substances typified by picrotoxinin, and 'neurotrophic' sesquiterpenes like jiadifenolide. Picrotoxinin elicits convulsion by anion blockade of the Cys-loop family of neurotransmitter-gated ion channels. These same receptors mediate neuronal development and neurite outgrowth prior to synapse formation. Due to its structural homology with picrotoxin and anisatin, it is possible that jiadifenolide enhances NGF-stimulated neurite outgrowth by modulation of the Cys-loop family of receptors.

Synthesis of Neurotrophic Seco-prezizaane Sesquiterpenes (1R,10S)-2-Oxo-3,4-dehydroneomajucin, (2S)-Hydroxy-3,4-dehydroneomajucin, and (-)-Jiadifenin

An asymmetric approach to the synthesis of neurotrophic seco-prezizaane sesquiterpenes is described that is based on the strategic application of a hydroxyl-directed metallacycle-mediated [2 + 2 + 2] annulation and an intramolecular radical cyclization cascade. Targets prepared are among the most potent members of the natural product class and include (1R,10S)-2-oxo-3,4-dehydroneomajucin, (2S)-hydroxy-3,4-dehydroneomajucin, and (-)-jiadifenin. In addition to representing the first application of the alkoxide-directed metallacycle-mediated hydrindane-forming annulation reaction in natural product synthesis and the first total synthesis of (2S)-hydroxy-3,4-dehydroneomajucin, these pursuits have resulted in the elucidation of a complex radical cascade process for installation of the C5 quaternary center common to the natural product class.

Protecting-Group-Free Total Synthesis of (-)-Jiadifenolide: Development of a [4 + 1] Annulation toward Multisubstituted Tetrahydrofurans

A concise, protecting-group-free total synthesis of (-)-jiadifenolide, a synthetically challenging seco-prezizaane sesquiterpene with potent neurotrophic activity, is reported. The convergent route features a SmI2/H2O-mediated stereoselective reductive cyclization, an unprecedented formal [4 + 1] annulative tetrahydrofuran-forming reaction and programmed redox manipulations. The newly developed annulation of β-hydroxy aldehydes or ketones with lithium trimethylsilyldiazomethane provides access to a diverse array of multisubstituted tetrahydrofurans. The synthetic jiadifenolide exhibited weak cytotoxicity against five human cancer cell lines.

Evolution of a Unified Strategy for Complex Sesterterpenoids: Progress toward Astellatol and the Total Synthesis of (-)-Nitidasin

Astellatol and nitidasin belong to a subset of sesterterpenoids that share a sterically encumbered trans-hydrindane motif with an isopropyl substituent. In addition, these natural products feature intriguing polycyclic ring systems, posing significant challenges for chemical synthesis. Herein, the evolution of our stereoselective strategy for isopropyl trans-hydrindane sesterterpenoids is detailed. These endeavors included the synthesis of several building blocks, enabling studies toward all molecules of this terpenoid subclass, and of advanced intermediates of our initial route toward a biomimetic synthesis of astellatol. These findings provided the basis for a second-generation and a third-generation approach toward astellatol that eventually culminated in the enantioselective total synthesis of (-)-nitidasin. In particular, a series of substrate-controlled transformations to install the ten stereogenic centers of the target molecule was orchestrated and the carbocyclic backbone was forged in a convergent fashion. Furthermore, the progress toward the synthesis of astellatol is disclosed and insights into some observed yet unexpected diastereoselectivities by detailed quantum-mechanical calculations are provided.

An eight-step gram-scale synthesis of (-)-jiadifenolide

Development of a biologically active secondary metabolite into a useful medicine requires continuous access to meaningful quantities of material. Although any chemical synthesis is broadly useful for its versatility, identification of a synthesis route that can be economically scaled represents a greater challenge. Here we report a concise synthesis of the neurotrophic trace metabolite (-)-jiadifenolide and its production on a gram-scale. The brevity of the route and the structural similarity of a key intermediate to many potent Illicium terpenes make chemical synthesis the unquestionable method for accessing and modifying these potential therapeutics.

Organocatalytic asymmetric strategies to carbocyclic structures by γ-alkylation-annulation sequences

Attractive carbocyclic structures are accessed via a highly regio- and enantioselective aminocatalytic γ-addition of cyclic enals to vinyl phosphonates followed by a one-pot intramolecular Horner-Wadsworth-Emmons reaction. It is also demonstrated that nitro olefins can act as electrophiles in a similar reaction concept, providing carbocycles in equally high stereoselectivity.

A lactone-fused cyclohexadiene as a versatile platform for diversified synthesis of 5,6,5-tricyclic scaffolds

A new lactone-fused cyclohexadiene platform for the stereoselective synthesis of differently functionalized 5,6,5-tricyclic scaffolds.

An annulation reaction for the synthesis of cross-conjugated triene-containing hydroindanes from acyclic precursors

In efforts directed toward the synthesis of seco-prezizaane sesquiterpenoids, a stereoselective annulation reaction has been developed between 4-hydroxy-1,6-enynes and TMS-alkynes that delivers cross-conjugated triene-containing hydroindanes. Contrary to previous reports, enyne substrates bearing two propargylic ethers enable the presumed organometallic intermediate to be trapped by double elimination. The tendency of products from this annulation to undergo Diels-Alder-based dimerization was harnessed to accomplish a two-step complexity-generating sequence en route to densely functionalized carbo- and heteorocyclic systems.

Neuritogenic surfaces using natural product analogs

Neuritogenic surfaces are generated by a simple dip-coating procedure, as glass slides are coated with a neurotrophin-like small organic molecule in the presence of a collagen matrix. The surfaces retain their biological activity for multiple cycles and the protocol is suitable for various substrates and coating conditions.

Total synthesis of jiadifenolide

As a potent neurotrophic agent, the sesquiterpenoid jiadifenolide represents a valuable small-molecule lead for the potential therapeutic treatment of neurodegenerative diseases. A stereocontrolled total synthesis of this densely functionalized natural product is reported, central to which is an adventurous samarium-mediated cyclization reaction to establish the tricyclic core and the adjacent C5 and C6 quaternary stereocenters.

An enantiospecific synthesis of jiadifenolide

A Robinson annulation, van Leusen homologation, and a desymmetrizing C-H oxidation enabled an enantiospecific synthesis of the neurotrophic natural product jiadifenolide. From a pulegone-derived building block, a key propellane intermediate was constructed through the use of simple reagents in a highly diastereoselective fashion. A short series of oxidations of this tricylic framework allowed progression to the natural product.

Neurotrophic natural products: chemistry and biology

Neurodegenerative diseases and spinal cord injury affect approximately 50 million people worldwide, bringing the total healthcare cost to over 600 billion dollars per year. Nervous system growth factors, that is, neurotrophins, are a potential solution to these disorders, since they could promote nerve regeneration. An average of 500 publications per year attests to the significance of neurotrophins in biomedical sciences and underlines their potential for therapeutic applications. Nonetheless, the poor pharmacokinetic profile of neurotrophins severely restricts their clinical use. On the other hand, small molecules that modulate neurotrophic activity offer a promising therapeutic approach against neurological disorders. Nature has provided an impressive array of natural products that have potent neurotrophic activities. This Review highlights the current synthetic strategies toward these compounds and summarizes their ability to induce neuronal growth and rehabilitation. It is anticipated that neurotrophic natural products could be used not only as starting points in drug design but also as tools to study the next frontier in biomedical sciences: the brain activity map project.

Metal-free aerobic C–H oxidation of cyclic enones

A metal-free procedure is described for the aerobic and complete C–H methylene oxidation of Hajos–Parrish enones to versatile dihydroindenediones.